yup.
all you need to make these headers on PC motherboards is the correct cable with the header plug at one end and the serial DB9 connector at the other end.
a few years ago all new PCs would come with this cable preinstalled but these days so few people have need of the serial port that it is often skipped.
you can still buy the completed cable in most shops that sell computer parts.
Interesting . . . I'm fairly sure you can directly hook the Tx pin on your microcontroller directly to that Rx pin on your mother board, and vice-versa.
nope. i'm afraid not.
the serial header pins on the motherboard are full spec RS232 so are correct for connecting to devices designed to plug into PC serial ports (like your AVR programmer)
not TTL UART devices (like the UART on an AVR).
dunk.